The present invention relates to a detection system for detecting X-ray radiation from a sample located in a microbeam instrument.
Microbeam instruments comprise a vacuum within which a sample that emits X-rays is located and include scanning electron microscopes (SEMs), transmission electron microscopes (TEMs), scanning transmission electron microscopes (STEMs), electron probe microanalysers (EPMAs) and defect review tools (DRTs).
In such detection systems, it is typically necessary to cool the sensor down to a temperature of around 90K. In the past, this has typically been achieved by connecting a large reservoir of liquid nitrogen to the sensor by a series of copper components. The copper components then operate to transfer heat from the sensor to the liquid nitrogen reservoir thereby maintaining the sensor at an operating temperature.
In recent years however, there has been a shift in design to replace the liquid nitrogen reservoir by a cooling device that is powered by electricity. Current commercial peltier cooling technology does not allow such low temperatures to be achieved. Accordingly candidate cooling devices typically use a compressor and thermodynamic cycle arrangement to achieve the low temperatures.
One such design is shown in GB-A-2325045. This describes an energy dispersive type semi-conductor (EDS) detector which is mounted inside a cryostat which is in turn slidably mounted to a system such as a scanning electron microscope (SEM). The cryostat is formed from a pulse tube refrigerator which operates to generate the low temperatures needed to cool the sensor. The pulse tube refrigerator is located at one end of the cryostat and is coupled to the sensor, located at the other end of the cryostat, by a cold finger. Heat is transferred along the cold finger from the sensor to the pulse tube cooler allowing the sensor to be maintained at the desired temperature.
However, this suffers from the major drawback that the sensor is cooled by the cold finger which results in a large loss in cooling efficiency. In particular, heat is generally absorbed along the entire length of the cold finger and accordingly, the pulse tube cooler must provide significantly more cooling power to ensure that the sensor is maintained at operating temperature. This has two main effects.
Firstly, a larger amount of energy is required to operate the system. Secondly, the compressor which drives the pulse tube cooling system must be of a sufficiently large size to obtain the cooling power. As a result, the compressor must be mounted separately to the pulse tube generator. In order to achieve this, at least one rotating valve is required to couple the compressor to the pulse tube which in turn leads to further losses in efficiency.
A second example of a system implementing a pulse tube generator is described in the Japanese publication JP-A-06109339. Again, in this example, a cold finger is used to connect the pulse tube refrigerator to the sensor to be cooled. Accordingly, this suffers from similar problems to those outlined above with respect to the system described in GB-A-2325045.
In accordance with the present invention, we provide a detection system for detecting X-ray radiation from a sample located in a microbeam instrument, the detection system comprising:
a. a pulse tube cooler;
b. a compressor connected to the pulse tube cooler;
c. a sensor coupled to the pulse tube cooler; and,
d. a housing containing the pulse tube cooler and the sensor,
wherein the pulse tube cooler, the sensor and at least part of the housing are sufficiently small to be positioned inside the microbeam instrument in use, thereby allowing the X-ray radiation from the sample to be detected by the sensor.
The invention provides a number of benefits. Firstly, by positioning the pulse tube cooler within the housing, it can be inserted into, typically the vacuum enclosure of, a microbeam instrument.
Secondly, positioning the sensor and the pulse tube cooler together inside the housing reduces the cooling effect required by the pulse tube cooler to keep the sensor at the desired temperature. This in turn improves the efficiency of the system allowing a smaller compressor to be used. The compressor can then be mounted on the instrument or even on the housing thereby further improving the efficiency of the cooling system in complete contrast to the conventional arrangements in which the compressor is floor mounted.
The conventional configurations for cooling sensors as described above in the introduction typically place a heat load on the cooler of approximately 2 watts. The present invention however by mounting the pulse tube cooler in the housing with the sensor can reduce the cooling power requirements from 2 watts to approximately 300 milliwatts.
The housing is preferably adapted to thermally insulate the sensor and the pulse tube cooler from the surroundings. Accordingly, the housing can be formed from a silvered thermally isolating material such as polished stainless steel to help further reduce heat loads on the sensor and the cooler.
Typically however, this is achieved by having the pulse tube cooler and the sensor maintained in a vacuum within the housing. The vacuum level is typically obtained at about 10xe2x88x926 millibars of pressure, thereby reducing heat transfer from the sensor and the pulse tube to the housing.
It is normally necessary to place the sensor as close to the sample as possible in order to achieve acceptable count rates in the detection system. Accordingly, when the detection system is mounted on to an SEM, it must be configured with the housing penetrating into the SEM with the compressor, which generates a large amount of heat, located outside the SEM housing. Accordingly, the housing preferably comprises an elongate outer tube having first and second ends, the sensor being positioned in the first end adjacent a window. This allows the sensor to achieve acceptable count rates.
In this situation, the pulse tube cooler is also usually positioned in the first end so that it can be directly coupled to the sensor.
The pulse tube cooler usually includes a cold heat exchanger with the sensor being attached to the cold heat exchanger via a short thermally conductive coupling. Minimizing the length of thermally conductive elements to couple the sensor to the cold heat exchanger increases the cooling efficiency of the system.
The pulse tube cooler is typically adapted to cool the sensor to a temperature below 150K, preferably below 100K, and if it is implemented as a multi-stage pulse stage cooler it could cool the sensor to a temperature of below 1K.
The sensor may be a superconductive tunnel type sensor or the like. Thus, this cooling system allows sensors such as Josephson Junction and Giaever Junction sensors, and microcalorimeters, such as transition edge sensors to be used.
As mentioned briefly above, an important application of the detection system is with an SEM. In this case the SEM usually includes a specimen chamber in which a sample to be imaged is positioned in use. In this case, the second end of the housing is coupled to a detector mounting on the SEM, the housing extending into the specimen chamber such that the first end is positioned adjacent the sample in use. In another case, the sample could be cut off from the rest of the specimen chamber by a thin walled xe2x80x9ctentxe2x80x9d of thin coated polymer for example. This xe2x80x9ctentxe2x80x9d would isolate the xe2x80x9ccleanxe2x80x9d vacuum, in the close vicinity of the sample and incident electron beam, from the potentially xe2x80x9cdirtyxe2x80x9d vacuum outside this region where peripheral detectors are inserted.